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Life on Mars, continued

This photomicrograph focuses on a large "biomorph" from a Mars meteorite fragment known as Nakhla e4150ed. Its chemical spectrum appears to be primarilyiron oxide but with a carbon content slightly greater than the underlying matrix.

Do rocks from Mars bear the tiny fossilized signs of life? Scientists who think so say they'll subject meteorites from the Red Planet to a new round of high-tech tests in hopes of adding to their evidence.

For years, only one meteorite has figured in the controversy: ALH84001, a rock that was blasted away from Mars 16 million years ago, floated through space and fell through Earth's atmosphere onto Antarctica about 13,000 years ago. Scientists reported in 1996 that the rock contained microscopic structures that looked like "nano-fossils," but skeptics said the structures could have been created by chemical rather than biological reactions.

In November, the scientists who were behind the earlier research reported fresh findings that they said answered many of the objections from the skeptics - and they said two other space rocks traced to Mars seemed to have "biomorph" structures similar to those found in ALH84001. Pictures of the biomorphs were spread across a couple of Web pages back then, but generated relatively little attention at the time.

Over the weekend, the Spaceflight Now Web site provided further details on what the scientists saw and what they think it means.

The team, headed by astrobiologist David McKay from NASA's Johnson Space Center, said that samples from ALH84001 and the two other meteorites - known as Nakhla (found in Egypt in 1911) and Yamato 596 (found in Antarctica in 2000) - would be analyzed with high-resolution electron microscopes as well as an ion microprobe system in the months ahead.

Such instruments are expected to provide much better information about the chemical composition of the samples - information that could show more definitively whether the processes giving rise to the biomorph structures were biological or strictly geological.

"We do not yet believe we have rigorously proven there is [or was] life on Mars," McKay told Spaceflight Now's Craig Covault. "But we do believe we are very, very close to proving there is or has been life there."

David McKay / NASA

A close-up view of a footprint-like pit in a Nakhla mineral. The cross lines in the pitare likely reflecting the fine-scale crystallographic structure of the underlying mineral revealed by differential dissolution by organic acid. The "toe" of this footprint still contains remains of the pit filling and is either microbial remains or precipitated clay minerals and oxides from the crack filling.

NASA could follow up on such findings with the Opportunity rover - which is due to start its seventh year on Mars this month. The search for signs of ancient life on Mars would be a job even more suited to the bigger, more capable Curiosity rover (a.k.a. Mars Science Laboratory), which is scheduled for launch in 2011. And if the evidence is really as strong as McKay hopes it will be, more Red Planet missions would likely be put on the fast track.

But there's always the chance that the evidence for life on Mars will remain inconclusive, even after the new, improved scientific tests. The Red Planet has been known to tease scientists before: You don't have to look any further than the Martian "canals" spotted in the 19th century, the Face on Mars photographed by the Viking 1 orbiter in 1976, the biology experiments conducted by the Viking landers, the Martian "banyan trees" touted by the late science-fiction guru Arthur C. Clarke, and the recurring reports about Martian methane.

Will biomorphs turn out to be the turning point in the search for life on Mars, or just another twist in a tangled tale? Feel free to weigh in with your comments below, and stay tuned for the next chapter in the "Life on Mars" story.

David McKay / NASA

Doughnut-shaped features dot this microscopic view of Nakhla sample 1151-1. It isnot clear whether these are single doughnut-shaped units or are two separate curved units with a space between them in the center. Note that these units mostly have spiky features at their margins.

Update for 9:30 p.m. ET: Several commenters have asked how we know that any of these space rocks came from Mars. The answer has to do with tiny pockets of gas that were found inside the meteorites. When the chemical makeup of the gas was analyzed, scientists discovered that it matched the unique signature of Martian atmosphere, as measured by the Viking landers back in the 1970s. ALH84001 was the first meteorite to be identified in this way, but other meteorites (including Nakhla and Yamato 596) have a similar signature. Check out this Web page for more about the signature of Martian atmosphere and other technical issues relating to ALH84001.

Even without the gas analysis, scientists can tell that the Mars meteorites are of alien origin. A few years ago, University of Hawaii planetary scientist Vicky Hamilton analyzed the mineral composition of ALH84001 and suggested that the rock was blown away from a region of Mars known as Eos Chasma, which is a branch of the planet's wide-ranging Valles Marineris canyon system. Valles Marineris would be a great place to look for life on Mars, if it weren't so darn hard to get down into.